EFFECT OF PRESSURE ON THE STRUCTURE OF MOLECULES

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EFFECT OF PRESSURE ON THE STRUCTURE OF MOLECULES

P. Sim, D. Klug, S. Ikawa, E. Whalley

To cite this version:

P. Sim, D. Klug, S. Ikawa, E. Whalley. EFFECT OF PRESSURE ON THE STRUC- TURE OF MOLECULES. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-173-C8-176.

�10.1051/jphyscol:1984831�. �jpa-00224331�

Colloque CS, supplément au n*ll, Tome if5, novembre 19Sf pageCS-173

EFFECT Of PRESSURE ON THE STRUCTURE OF M O L E C U L E S *

P.G. Sim, D.D. Klug, S. Ikawa and E. WhaLley

Division of Chemistry^ National Research Council, Ottawa K1A 0R9, Canada

Résumé - On a déterminé pat spectroscopie infrarouge l'effet de la pression sur les angles de rotation internes du gauche-1,2-dichlorétKane et du

s-trithiane. L'angle de rotation interne du gauche-l,2-dichloréthane varie au taux de -2,7° kbar jusqu'à 5 kbars, et la molécule de s-trithiane devient plate sous une pression d'environ 50 kbars.

Abstract - The effect of pressure on the internal rotation angles of gauche-1,2-dichloroethane and s-trithiane has been determined by infrared spectroscopy. The internal rotation angle of gauche-1,2-dichlocoethane changes at the rate of -2,7° kbar-1 up to 5 kbar, and s-trlthiane becomes a flat molecule at ~50 kbar.

1 - INTRODUCTION

The sizes and shapes of molecules are part of the foundations of chealstry: all our understanding of chemical equilibria and chemical reaction rates depends on what we know about thera. The effects of low pressures on chemical equilibria and reaction rates are determined by standard volume changes iV* or volumes of activation AV at low pressure and so depend on the sizes and shapes of molecules at low pressure. The effect of pressure on equilibrium and rate constants at high pressures depends, similarly, on the sizes and shapes of molecules at high pressures. Chemical reaction rates have been measured up to 80 kbar /!/ (although the preliminary interpretation of the results requires some modification), and so a knowledge of the effects of pressure on molecular sizes and shapes is becoming important for understanding chemistry at very high pressure.

The structure of a molecule can be described by Its configuration, that is, a

description of the chemical bonding in terms of the topology and the bond lengths and bond angles, and its conformation, that is, a description of the angles of internal rotation about single bonds. All aspects of the structure of a molecule can be changed by pressure, but the largest changes will often occur to the internal rotation angles because the forces preventing Internal rotation are usually much smaller than those preventing changes of bond lengths and bond angles. Internal rotations are, of course, very important in many aspects of chemistry: the flexibility of natural and synthetic polymers depends on thera, and internal rotations control the behaviour of proteins, DNA, etc., and the effect of pressure on them should help us to understand their behaviour. Molecular conformations respond to pressure by changing the equilibrium between conformational isomers, the angle of internal rotation, and the fluctuation of the angle.

The effect of pressure on the Internal rotation angle of trithiane /2/, a puckered six-me inhered ring compound (CH2S)3, analogous to trloxane, and gauche-1,2-

dichloroethane and gauche-1,2-dlbromoethane /3/ have already been determined by Raman

*N.R.C. No. 23716

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984831

C8-174 JOURNAL DE _PHYSIQUE

s p e c t r o s c o p y up t o 1 8 , 5 , and 3 k b a r r e s p e c t i v e l y . The t r a n s - g a u c h e e q u i l i b r i u m of 1 , 2 - d i c h l o r o e t h a n e and 1 , 2 - d i b r o m o e t h a n e / 4 / and t h e f l u c t u a t i o n of t h e i n t e r n a l r o t a t i o n a n g l e of t r a n s - l , 2 - d i b r o m o e t h a n e / 3 / have a l s o been measured. T h i s work h a s been e x t e n d e d t o 1 , 2 - d i c h l o r o e t h a n e up t o 30 k b a r / 5 / and t o t r i t h i a n e u p t o 1 0 0 k b a r / 6 / u s i n g i n f r a r e d s p e c t r o s c o p y .

I1

-

INTERNAL ROTATION ANGLE I N GAUCHE-1,2-DICHLOROETHANE

The i n t e r n a l r o t a t i o n a n g l e of gauche-1,2-dichloroethane, C1CH2CH2C1, which i s t h e d i h e d r a l a n g l e between t h e C1-C-C and C-C-C1 p l a n e s , d e t e r m i n e s t h e r e l a t i v e i n f r a r e d i n t e n s i t y o f t h e i n - p h a s e and o u t - o f - p h a s e C-C1 s t r e t c h i n g bands. I f t h e two

v i b r a t i o n s do n o t c o u p l e s i g n i f i c a n t l y t o o t h e r m o t i o n s , t h e r a t i o is

where a i s t h e C-C-C1 a n g l e and 0 t h e i n t e r n a l r o t a t i o n a n g l e . An a p p r o x i m a t e

a l l o w a n c e f o r c o u p l i n g t o o t h e r modes and f o r o t h e r e f f e c t s c a u s e s t h e r i g h t - h a n d s i d e of Eq. ( 1 ) t o be m u l t i p l i e d by a c o n s t a n t , whose v a l u e was o b t a i n e d a s 0.53 from t h e measured r a t i o of i n t e n s i t i e s a t z e r o p r e s s u r e and t h e e q u i l i b r i u m g e o m e t r y i n t h e v a p o r , i.e. a

-

l o g 0 , B = 76.4' /7/. The r e l a t i v e i n f r a r e d i n t e n s i t y i s , t h e r e f o r e , a m e a s u r e of t h e i n t e r n a l r o t a t i o n a n g l e i f t h e C-C-C1 a n g l e i s known.

S o l u t i o n s o f 1 , 2 - d i c h l o r o e t h a n e i n 2 - m e t h y l b u t a n e c o n t a i n i n g 1.7, 5, and 10% v / v w e r e h e l d i n a d i a m o n d - a n v i l c e l l u s i n g type-304 s t a i n l e s s - s t e e l g a s k e t s , 5 0 , 80, o r 250 pm t h i c k w i t h a 360-pm h o l e . The s p e c t r a were measured a t 2Z°C w i t h a N i c o l e t 7199 M i c h e l s o n i n t e r f e r o m e t e r w i t h a s p e c t r a l r e s o l u t i o n o f 2 cm-'. The p r e s s u r e was measured t o f-1 k b a r by t h e f r e q u e n c y of t h e s t r o n g 2 - m e t h y l b u t a n e band a t 764 cm-l, w h i c h was i n t u r n c a l i b r a t e d a g a i n s t t h e a s y m m e t r i c s t r e t c h of a d i l u t e s o l u t i o n of sodium n i t r i t e i n sodium bromide / 8 / , and changed l i n e a r l y w i t h p r e s s u r e i n t h e r a n g e 0-36 k b a r a t t h e r a t e of 0.51t0.05 cm-l k b a r - l .

The i n f r a r e d C-C1 s t r e t c h i n g bands of t h e g a u c h e c o n f o r m e r a r e p l o t t e d i n F i g . 1 a t z e r o and 9.6 k b a r . The r e l a t i v e i n t e n s i t y of t h e bands of 5 and

b

symmetry c l e a r l y i n c r e a s e s g r e a t l y , and t h e r a t i o of t h e i r i n t e n s i t i e s i s p l o t t e d up t o 30 k b a r i n F i g . 2. By c o m b i n i n g t h e d a t a of F i g . 2 w i t h Eq. ( I ) , t h e i n t e r n a l r o t a t i o n a n g l e c h a n g e s a t t h e r a t e -2.7" k b a r u p t o -5 k b a r , i . e . t o 63O, and t h e r e l a t i v e i n t e n s i t y r e m a i n s c o n s t a n t a t h i g h e r p r e s s u r e s . A s t h e r e l a t i v e i n t e n s i t y m e a s u r e s d i r e c t l y t h e a n g l e between t h e two C-Cl b o n d s , t h i s a n g l e seems t o r e m a i n c o n s t a n t between 5 and 30 k b a r . I t seems t h a t when t h e i n t e r n a l r o t a t i o n a n g l e d e c r e a s e s below 63" t h e r e p u l s i o n b e t w e e n t h e two c h l o r i n e a t o m s o p e n s t h e C-C-C1 bond enough t o k e e p t h e a n g l e b e t w e e n t h e C-C1 bonds e q u a l .

Fig. 1 The i n f r a r e d C-C1 s t r e t c h i n g bands of a 5% v / v s o l u t i o n of gauche-1,2- d i c h l o r o e t h a n e i n 2 - m e t h y l b u t a n e s o l u t i o n a t 0 a n d 9.6 k b a r .

111 - FLATTENING OF TRITHIANE

The i s o l a t e d p u c k e r e d t r i t h i a n e m o l e c u l e h a s symmetry C3v. I t h a s s e v e n A1 b a n d s ,

-

i o . [ / x ~

0.6L

.

I I I I I

0 2 4 6 8 1 0 2 0 30

p/ kbar

F i g . 2 K a t i o of t h e r e l a t i v e i n t e n s i t i e s of t h e 5 and

b

C-C1 s t r e t c h i n g v i b r a t i o n s o f gauche-l,2-dichloroethane i n s o l u t i o n i n 2 - m e t h y l b u t a n e a t 22OC.

f o u r of which s h o u l d l o s e i n f r a r e d a c t i v i t y when i t f l a t t e n s t o a D3h m o l e c u l e , and t e n E b a n d s , t h r e e of which s h o u l d l o s e i n f r a r e d a c t i v i t y on f l a t t e n i n g . The Raman s p e c t r u m c a n , t h e r e f o r e , be u s e d t o d e t e c t f l a t t e n i n g .

S p e c t r a of a d i s p e r s i o n of t r i t h i a n e i n sodium b r o m i d e , h e l d i n a d i a m o n d - a n v i l c e l l , w e r e r e c o r d e d a t 2-cmdl n o m i n a l r e s o l u t i o n w i t h a N i c o l e t 7199 M i c h e l s o n i n t e r f e r o - m e t e r u s i n g sodium n i t r i t e i n sodium bromide a s a p r e s s u r e g a u g e / 8 / .

Of t h e s e v e n b a n d s t h a t a r e e x p e c t e d t o d i s a p p e a r on f l a t t e n i n g t o a D3h m o l e c u l e , two a r e below o u r f r e q u e n c y r a n g e and t h e A1 CH2 s y m m e t r i c s t r e t c h c a n n o t be r e s o l v e d w e l l e n o u g h from t h e E f o r a q u a n t i t a t i v e s t u d y . The i n t e n s i t i e s of t h e f o u r r e m a i n i n g b a n d s , t h e A v l r i n g s t r e t c h a t 654 cm-l, E CH2 r o c k a t 729 cm-l, and E CH2 t w i s t a t 1217 and 122: cm-l a t z e r o p r e s s u r e , w h i c h were measured r e l a t i v e t o t h e Al CH2 r o c k a t 9 0 8 cm-l, a r e p l o t t e d i n F i g . 3. The r a p i d l o s s of r e l a t i v e i n t e n s i t y i n t h e f i r s t -30 k b a r is l i k e l y due t o f l a t t e n i n g , and t h e more s l o w l y v a r y i n g i n t e n s i t y a t h i g h e r

p r e s s u r e s a r i s e s from d i s t o r t i o n from D3h symmetry, a n d , f o r t h e 654 cm-l b a n d , f r o m t h e r e s i d u a l i n t e n s i t y of t h e 662 cm-' band, which c a n n o t be r e s o l v e d from i t .

F i g . 3 Graph of t h e a r e a s of t h e 654 (Al v r i n s t r e t c h )

+

^{662 (E} r i n g b e n d ) , 796 ( E r i n g s t r e t c h ) , and 1 2 1 1 + 1 2 2 5 (E CH2 t x i s t j cm-q b a n d s r e l a t i v e t o t h a t of t h e 908-cm-' (A1 CH2 r o c k ) a g a i n s t t h e p r e s s u r e . Frame c i s n o t r e l e v a n t h e r e . The d e v i a t i o n of t h e c u r v e s from t h e e x t r a p o l a t e d h i g h - p r e s s u r e p a r t of t h e l i n e s , w h i c h a r e drawn a s d a s h e d c u r v e s i n F i g . 3 , were a n a l y z e d by a s s u m i n g b o n d - d i p o l e models f o r t h e i n t e n s i t y of t h e bands a s a f u n c t i o n o f t h e d e g r e e of p u c k e r i n g , w h i c h i s measured by t h e q u a n t i t y $ = x / a w h e r e x is t h e d i s t a n c e between t h e c a r b o n and s u l p h u r p l a n e s a n d a t h e C-S bond l e n g t h . A t z e r o p r e s s u r e , @ = $o i s 0.359 / Y / . The i n t e n s i t i e s , r e l a t i v e t o t h e A 1 CH2 r o c k , of t h e t h r e e bands t h a t c a n be a n a l y z e d

C8-176 JOURNAL DE PHYSIQUE

q u a n t i t a t i v e l y a r e on t h i s model: E CH2 r o c k 6 . 7 5 9 @ 2 / ( 1 - @ 2 ) , A L vl r i n g s t r e t c h 4 . 8 7 4 @ 2 ( 1 + 3 @ 2 ) / ( 1 - $ 2 ) , and E CH2 t w i s t 5.596 $2(1+3@2).

The v a l u e s o f $I a n d 0 = $ / @ O where $o i s t h e v a l u e of $ a t z e r o p r e s s u r e , a s o b t a i n e d f r o m t h e s e e x p r e s s i o n s and t h e c u r v e s i n F i g . 3 , a r e p l o t t e d a g a i n s t t h e p r e s s u r e i n F i g . 4. A l t h o u g h t h e t h r e e b a n d s d o n o t a g r e e e x a c t l y , no d o u b t b e c a u s e t h e t h e o r y i s a p p r o x i m a t e , t h e y a l l i n d i c a t e f l a t t e n i n g i n t h e r a n g e 40-60 k b a r . The e a r l i e r Raman m e a s u r e m e n t s / 2 / a r e a l s o p l o t t e d i n Fig. 4 a s a d a s h e d l i n e ; t h e y a g r e e w e l l w i t h t h e i n f r a r e d r e s u l t s . T h e r e i s , t h e r e f o r e , l i t t l e d o u b t t h a t t r i t h i a n e becomes f l a t a t a b o u t 50 k b a r , and a l l t h e bonds a r e t h e n i n t h e c i s c o n f o r m a t i o n .

- -. - - - . . . - - - -. - . -

o E CH2 ROCK. 7 2 9 cm-'

+' x E CHI TWIST. 1225.1217<~,

+ + + 1

0 7 -

I + ^{+ . .}

⁺

^.

F i g . 4 Graph of 8

-

^{@/@, =} x / x o a n d @ a s a f u n c t i o n of p r e s s u r e f o r t h e t h r e e i n f r a r e d b a n d s t h a t become i n a c t i v e i n t h e D t r i t h i a n e m o l e c u l e . The u p p e r s o l i d l i n e s a r e drawn t h r o u g h t h e p o i n t s d e t e r m i n e a h b y t h e E CHz r o c k and E CH2 t w i s t , and t h e l o w e r i s drawn t h r o u g h t h e p o i n t s d e t e r m i n e d by t h e A1 r i n g s t r e t c h . The d a s h e d l i n e was o b t a i n e d from t h e Kaman s p e c t r u m up t o 17.8 k b a r / 2 / .

I V

-

DISCUSSION

T h i s and e a r l i e r / 2 , 4 / work shows c l e a r l y t h a t t h e i n t e r n a l r o t a t i o n a n g l e s o f m o l e c u l e s c a n be changed s i g n i f i c a n t l y by p r e s s u r e , and t h a t t h e c h a n g e s c a n be measured by t h e i n f r a r e d and Raman s p e c t r u m . The most s p e c t a c u l a r r e s u l t i s t h e c o m p l e t e f l a t t e n i n g of t r i t h i a n e a t -50 k b a r . R e c e n t work t h a t is n o t y e t f u l l y a n a l y z e d / l I / shows t h a t t r i s e l e n a n e , t h e s e l e n i u m a n a l o g u e of t r i t h i a n e , a l s o becomes f l a t u n d e r p r e s s u r e .

The c h a n g e s of i n t e r n a l r o t a t i o n a n g l e s h o u l d h a v e i m p l i c a t i o n s f o r c h e m i s t r y t h a t have n o t y e t been e x p l o r e d . I n f l a t t r i t h i a n e , f o r e x a m p l e , t h e d i s t i n c t i o n b e t w e e n a x i a l and e q u a t o r i a l C-H bonds i s l o s t . The c h a n g e s s h o u l d a l s o h a v e i m p l i c a t i o n s F o r r e a c t i o n r a t e s u n d e r p r e s s u r e , and s h o u l d be r e l e v a n t t o c h e m i c a l s y n t h e s i s .

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